This invention relates to a voltage-controlled oscillator circuit used, for example, in a phase-locked loop (PLL) in a coding-decoding device (CODEC) in a digital telephone switching system.
A voltage-controlled oscillator produces an output signal .phi. having a frequency f that depends on an applied control voltage, the dependence being substantially linear. As the control voltage varies in a control range V.sub.c, the frequency f varies in a corresponding frequency range f.sub.c. In applications such as the one mentioned above, the purpose of the voltage-controlled oscillator circuit is to hold the frequency f constant at certain value f.sub.0. The frequency range f.sub.c must therefore contain the value f.sub.o.
The process of manufacturing a voltage-controlled oscillator, however, is attended by unavoidable variability, which leads to variations in capacitance values, transistor transconductance values, and other circuit parameters. These variations displace the frequency range f.sub.c by variable amounts in the upward or downward direction. The circuit designer is faced with the problem of ensuring that, despite such variations, the frequency range f.sub.c corresponding to the control range V.sub.c still includes the target frequency f.sub.0.
A prior-art solution to this problem has been to widen the frequency range f.sub.c by increasing the voltage-to-frequency conversion gain. Unfortunately, this solution has undesirable side effects. In a PLL, for example, the increased gain widens the loop bandwidth, making the PLL susceptible to external noise effects and liable to jitter.